This application claims the priority benefit of Taiwan application serial no. 89116721, filed Aug. 18, 2000.
1. Field of Invention
The present invention relates to a mass storage medium. More particularly, the present invention relates to an optical recording medium structure and method of manufacture.
2. Description of Related Art
Through the popularization of multimedia, large quantities of textual, sound and image data are generated. One consequence of this is the rapid development of data storage medium, especially in the storage density and access rate of the medium. Due to the advantages including high storage capacity, stability over long period, low production cost, high compatibility and low error rate, compact disk (CD) is one of the major storage medium for multimedia system. Amongst the variety of compact disk types, writable type of optical disks are now widely used. To write data onto a writable disk, a beam of laser is shone onto a recording material to change the optical property of recording material at a particular location. Data value at the location can also be read out through laser beam reflection. For example, phase transition type of writable disk utilizes the difference in amount of reflection of a laser beam between the crystalline state and the non-crystalline state of a recording material to record digital data. Because old data can be erased and new data can be rewritten onto a phase transition type of writable disk concurrently by modulating laser beam power, phase transition type of writable disk is a high-speed recording medium.
FIG. 1 is a schematic cross-sectional view showing a conventional phase transition type of optical disk. As shown in FIG. 1, the phase-transition optical disk 10 consists of a stack of layers. The stack includes, from bottom to top, a substrate 12, a first dielectric layer 14, a recording layer 16, a second dielectric layer 18, a reflection layer 20 and a passivation layer 22. The first dielectric layer 14 and the second dielectric layer 18 is made from a material such as silicon oxide (SiOx) or zinc sulfide-silicon dioxide (ZnSxe2x80x94SiO2). The recording layer 16 is an alloy material that includes germanium (Ge), tellurium (Te) and antimony (Sb).
In general, the stochiometric composition of germanium, tellurium and antimony inside the recording layer 16 is Ge2Sb2Te5. However, as wavelength of laser beam used in optical disk read/write is shortened, difference in optical coefficients between a recording material in a non-crystalline state and one in a crystalline state becomes smaller. In other words, difference in optical reflection between the two optical states becomes smaller rendering the differences in digital levels more difficult to decipher. To facilitate digital signal reading, the standard stochiometric composition of Ge2Sb2Te5 is ultimately relaxed so that a recording layer having a higher ratio of germanium and tellurium is used.
Although using a recording material with a higher composition of germanium and tellurium is able to increase signal modulation, repeatability of the recording medium will deteriorate due to straying from the standard stochiometric composition. The main reason is that a relatively high temperature is required for rewriting new data. After re-writing a few times, the sulfur atoms and oxygen atoms within the second dielectric layer 18 and the first dielectric layer 14 will diffuse into the recording layer 16. Similarly, germanium, antimony and tellurium atoms within the recording layer 16 will also diffuse into the first and second dielectric layer 14 and 18 respectively.
In addition, the coefficient of absorption between the crystalline region and non-crystalline regions (representing recording marks of digital data) of the recording layer 16 are different. Hence, the recording marks inside the recording layer 16 may be distorted after some re-writing leading to slight jittering of the signal and poorer data read-off. In brief, a lowering of the wavelength of laser beam to increase recording density leads to a recording layer having a composition that deviates from the standard stochiometric composition. Hence, there is a need to find a method capable of preventing the diffusion of atoms between the recording layer and surrounding dielectric layers so that distortion of recording marks can be avoided even after repeated re-writing operations.
Accordingly, one object of the present invention is to provide an optical recording medium structure capable of slowing signal deterioration due to repeated re-writing operations. The optical recording medium structure includes a transparent substrate, a first dielectric layer over the substrate, a first buffer layer over the first dielectric layer, a recording layer over the first buffer layer, a second buffer layer over the recording layer, a second dielectric layer over the second buffer layer, an optical compensation layer over the second dielectric layer and a reflection layer over the optical compensation layer. The recording medium structure may further include an additional passivation layer over the reflection layer for damage protection.
The invention also provides a method of forming an optical recording structure. A transparent substrate is provided. A first dielectric layer, a first dielectric layer, a first buffer layer, a recording layer, a second buffer layer, a second dielectric layer, an optical compensation layer and a reflection layer are sequentially formed over the substrate. An additional passivation may form over the reflection layer for damage protection.
In the embodiment of this invention, the first (the second) buffer layer serves to prevent the diffusion of atoms of at least one element type in the recording layer into the first (the second) dielectric layer. The first (the second) buffer layer also serves to prevent the diffusion of atoms of at least one element type in the first (the second) dielectric layer into the recording layer. The optical compensation layer serves to enhance thermal sensitivity of the recording layer during an optical read/write operation. In addition, spiraling guide grooves may also form on the transparent substrate for collimating laser beam for more accurate reading. Furthermore, a phase transition material capable of transiting reversibly between a crystalline state and a non-crystalline state may be used to form the recording layer.
In this invention, the recording layer is sandwiched between two buffer layers so that inter-diffusion of atoms between the recording layer and the first (the second) dielectric layer is avoided and a constant composition of the recording layer is maintained. Since a shorter wavelength laser beam must be used to increase storage capacity of optical disk, non-standard stoichiometric composition must be used to form the recording layer. This invention can effectively stop the inter-diffusion of atoms between the recording layer and the first (the second) dielectric layer after repetitive re-writing and hence can prevent the gradual deterioration of the recording medium. In addition, an optical compensation layer is used to enhance thermal sensitivity of the recording layer during an optical read/write operation. Since the light-absorption capacity in the crystalline regions is greater than light-absorbing capacity in the non-crystalline regions, distortion of recording marks is minimized.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.